DONALD J. TRUMP, until now a Republican problem, this week became a challenge the nation must confront and overcome. The real estate tycoon is uniquely unqualified to serve as president, in experience and temperament. He is mounting a campaign of snarl and sneer, not substance. To the extent he has views, they are wrong in their diagnosis of America’s problems and dangerous in their proposed solutions. Mr. Trump’s politics of denigration and division could strain the bonds that have held a diverse nation together. His contempt for constitutional norms might reveal the nation’s two-century-old experiment in checks and balances to...